At present, air is commonly cooled by air conditioners that employ energy consuming refrigeration machines. These machines are of a relatively complicated design, and are mechanically unreliable and costly to operate. In many instances, the simpler alternative technique of evaporative cooling can be used for cooling purposes.
Many methods and apparatus are known that employ evaporative processes for cooling various fluids. In particular, evaporative processes are often used to cool atmospheric air. These processes exploit the various physical properties of the atmosphere for extracting heat therefrom.
The atmosphere is a mixture of air and water vapor. The temperature of the atmosphere, as indicated by a normal thermometer, is the dry-bulb temperature (td). If the atmosphere is cooled under a constant total pressure, the partial pressures remain constant until a temperature is reached at which condensation begins. This temperature is the dew point (tc) or condensation temperature. Heating and cooling above the dew point entails no condensation of vapor. If the bulb of a thermometer is covered with absorbent material, e.g., linen, wet with distilled water, and exposed to the atmosphere, evaporation will cool the water and the thermometer to the wet-bulb temperature (tw). This is the temperature given by a psychrometer. The wet-bulb temperature lies between the drybulb temperature and the dew point. The difference between the wet-bulb temperature and the dry-bulb temperature is the psychometric differential. These three temperatures, the drybulb temperature, the wet-bulb temperature and the dew point, are distinct from each other except in an atmosphere saturated with water vapor, in which they are identical. The drier the atmosphere, the more these temperatures are different from one another.
Moisture can be absorbed by a body of air until that body of air becomes saturated. When moisture evaporates from a surface and is absorbed by a body of air, the surface from which the moisture evaporates is cooled. This is called evaporative cooling. The cooled surface can then be used to cool something else, for example, to cool another body of air. This is achieved by extracting heat from that body of air.
However, evaporate cooling techniques are limited by the moisture absorbing capacity of the ambient air. Thus, in very humid climates, the amount of moisture that the ambient air can absorb, and thus cool a surface, is limited. Conversely, in those limited climatic zones that have ambient air with an exceedingly low moisture content, evaporative cooling is a low energy, economically feasible alternative to air conditioning. By exploiting the physical thermal characteristics of the atmosphere, the ambient air can be cooled, and comfort enhanced.